The sympathetic nervous system (SNS) is a primarily involuntary bodily control system typically associated with stress responses. SNS fibers that innervate tissue are present in almost every organ system of the human body and can affect characteristics such as pupil diameter, gut motility, and urinary output. Such regulation can have adaptive utility in maintaining homeostasis or preparing the body for rapid response to environmental factors. Chronic activation of the SNS, however, is a common maladaptive response that can drive the progression of many disease states. Excessive activation of the renal SNS in particular has been identified experimentally and in humans as a likely contributor to the complex pathophysiologies of hypertension, states of volume overload (such as heart failure), and progressive renal disease.
Sympathetic nerves innervating the kidneys terminate in the blood vessels, the juxtaglomerular apparatus, and the renal tubules. Stimulation of the renal sympathetic nerves can cause, for example, increased renin release, increased sodium reabsorption, and reduced renal blood flow. These and other neural-regulated components of renal function are considerably stimulated in disease states characterized by heightened sympathetic tone and likely contribute to increased blood pressure in hypertensive patients. For example, reduced renal blood flow and glomerular filtration rate as a result of renal sympathetic efferent stimulation is likely a cornerstone of the loss of renal function in cardio-renal syndrome (i.e., renal dysfunction as a progressive complication of chronic heart failure). Pharmacologic strategies to thwart the consequences of renal efferent sympathetic stimulation include centrally acting sympatholytic drugs, beta blockers (intended to reduce renin release), angiotensin converting enzyme inhibitors and receptor blockers (intended to block the action of angiotensin II and aldosterone activation consequent to renin release), and diuretics (intended to counter the renal sympathetic mediated sodium and water retention). These pharmacologic strategies, however, have significant limitations including limited efficacy, compliance issues, side effects, and others.
Recently, neuromodulation devices have been developed that may reduce sympathetic nerve activity by applying an energy field to a target site in the renal blood vessel (e.g., via radio frequency ablation) and may thereby reduce blood pressure in patients with treatment-resistant hypertension. A number of neuromodulation devices utilize one or more thermocouples (“TC”) to measure temperature at or near an electrode. The presence of TC wires within the catheter shaft, particularly in neuromodulation devices having two or more electrodes, can make the shaft bulky, stiff, and/or relatively expensive to manufacture.